Evaluation of Mechanical Properties and Comprehensive Modeling of CMC with Stiff and Weak Matrices

Abstract

The mechanical properties of ceramic matrix composites (CMC) depend on the
individual properties of fibers and matrix, the fiber-matrix interface, the microstructure and the
orientation of the fibers. The fiber-matrix interface of ceramics with stiff matrices (e.g. CVI-derived
SiC/SiC) must be weak enough to allow crack deflection and debonding in order to achieve
excellent strength and strain to failure (weak interface composites WIC). This micromechanical
behavior has been intensively investigated during the last 20 years. With the development of CMC
with weak matrices (weak matrix composites WMC) as e.g. oxide/oxide composites or polymer
derived CMC the mechanical response can not be explained anymore by these models as other
microstructural mechanisms occur. If the fibers are oriented in loading direction in a tensile test the
WMC behave almost linear elastic up to failure and show a high strength. Under shear mode or if
the fibers are oriented off axis a significant quasiplastic stress-strain behavior occurs with high
strain to failure and low strength. This complex mechanical behavior of WMC will be explained
using a finite element (FE) approach. The micromechanical as well as the FE models will be
validated and attributed to the different manufacturing routes.